Surface plasmon lens for heat assisted magnetic recording

ABSTRACT

An apparatus for focusing plasmon waves to a spot. The plasmon waves are there converted to light. In one application, the light is used for heat induced magnetic recording. In another application, the light is used as a part of near field scanning microscope. The plasmon waves may be induced on a converging rectangular cone having an aperture. The plasmon waves may also be focused on a flat surface by a curved dielectric lens. In the heat induced magnetic recording embodiment, a magnetic pole structure is integrated into the focusing apparatus, either as one surface of the rectangular cone, or as a layer upon which the curved dielectric lens is formed.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims benefit of U.S. provisional patentapplication serial No. 60/346,378, filed on Jan. 7, 2002, No.60/346,379, filed on Jan. 7, 2002, and No. 60/346,431, filed on Jan. 7,2002, which are herein incorporated by reference.

[0002] BACKGROUND OF THE INVENTION

[0003] 1. Field of the Invention

[0004] The invention relates to the field of near field optics and moreparticularly to its use in heat assisted magnetic recording.

[0005] 2. Description of the Related Art

[0006] Heat-assisted magnetic recording (HAMR) involves heating a spoton the disk surface to reduce its coercivity sufficiently so that it canbe magnetically recorded. The advantage of this technique is that thecoercivity of the media at ambient can be significantly increased,thereby improving thermal stability of the recorded data even for verysmall bit cells. One of the difficulties with the technique is finding amethod to heat just the small area of media which is to be recorded.Heating with laser light, as is done in magneto-optic recording, is themost promising approach, but the difficulty with this is that at thecurrent storage densities contemplated for HAMR, the spot to be heatedis ˜25 nm in diameter, which is fifty times smaller than the wavelengthof useful semiconductor lasers. The so-called diffraction limit inoptics is the smallest dimension to which a light beam can be focused.The diffraction limit in three dimensions is given by the equation$\begin{matrix}{d = {- \frac{0.6\lambda}{n\quad \sin \quad \theta}}} & (1)\end{matrix}$

[0007] where d is the spot diameter, λ is the wavelength of the light infree space, n is the refractive index of the lens, and θ is the maximumangle of focused light rays from the central axis of the lens. Thefactor l/n is the wavelength of the light within the lens. The spotdiameter is directly proportional to the wavelength of the light withinthe lens. The minimum focused spot diameter in the classical diffractionlimit is ˜λ/2, which is much too large to be useful for HAMR.

[0008] When light is incident upon a small circular aperture, it iswell-known in classical optics that the amount of power transmittedthrough the aperture scales as the ratio of the aperture to thewavelength raised to the fourth power [H. A. Bethe, “Theory ofDiffraction by Small Holes” Phys. Rev. 66 (1944) 163-182]. In otherwords, the amount of light which can be transmitted through an aperturewith a ˜25 nm diameter at a wavelength of 500 nm is ˜6×10−6 of theamount that would be expected for the size of the hole. This throughputis orders of magnitude too small to be practical for HAMR.

[0009] Therefore, there is a need to focus or confine energy from alight source having a wavelength on the order of 500 nm or greater intoa spot whose diameter is on the order of 25 nm with high transmissionefficiency. The relevant art provides no solution.

[0010] SUMMARY OF THE INVENTION

[0011] The present invention comprises apparatus that generates anintense, but very small, radiating source of light by efficientlyconverting an incident light beam into a surface plasmon wave, bringingthe surface plasmon wave to a tight focus in a structure for which thesurface plasmons have a very small wavelength, and then converting theenergy in the surface plasmon back into light at the focus.

[0012] In one embodiment, light is incident onto a metal/dielectricinterface where it induces a plasmon wave that travels in the samedirection as the incident beam. The plasmon waves then encounter adielectric lens that focuses the light to a small-diameter spot on aflat surface. There, the plasmon wave converts back into light as itexits the lens. There the light may be used as part of a scanningmicroscope or to heat a nearby magnetic recording medium.

[0013] The structure may include an integrated magnetic pole for use ina disk drive. The pole is aligned on the focused spot and may either bein the form of a narrow shaft or a paddle that narrows to a tipco-located with the focus of the plasmon wave.

[0014] The lens structure may be formed of a curved, high dielectricmaterial. Alternatively, the metal layer may be provided with a regionof different thickness curved to refract the plasmon wave to the focusedspot.

[0015] In an another embodiment, the plasmon waves are excited in acone-type metal/dielectric structure that narrows to an apex. The metallayer is the outermost layer to confine the plasmon waves. The metallayer is removed from the apex, allowing the plasmon waves to there beconverted back into light.

[0016] The cone may be rectangular and the incident light beam may bepolarized to excite the plasmon wave in the longer surfaces of the cone.In a variation used for magnetic recording, one of the longer surfacesadds or replaces the dielectric/metal layers with the magnetic pole of adisk drive.

[0017] The present invention is expected to have a wide range ofapplications, not just for HAMR, but also in the emerging fields ofmicrooptics and near-field scanning optical microscopy.

[0018] BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is the illustration of a lens focusing a light beam on aspot.

[0020]FIG. 2 is a representation of a bi-thickness metallic layerconducting and diffracting a plasmon wave.

[0021]FIG. 3a is a perspective view of a plasmon wave lens according toan embodiment of the invention.

[0022]FIG. 3b is a cross-sectional view of the lens of FIG. 3a.

[0023]FIG. 4 is a chart of the curvature of the lens surface of thefirst embodiment.

[0024]FIG. 5 is a perspective view of a plasmon wave lens according toanother embodiment of the invention that includes a magnetic pole.

[0025]FIG. 6 is a perspective view of a plasmon wave lens according toanother embodiment of the present invention that includes a taperedmagnetic pole.

[0026]FIG. 7 is a perspective view of a plasmon wave lens according toanother embodiment of the present invention that includes a taperedmagnetic pole forming one surface of half-lens embodiment.

[0027]FIG. 8 is an illustration of the essential layers of a plasmonwave focusing probe structure according to other embodiments of thepresent invention.

[0028]FIG. 9 is a cross sectional view of another embodiment of theinvention that is structured in the shape of a cone.

[0029]FIG. 10 is a chart of plasmon resonance vs. beam angle ofincidence for gold.

[0030]FIG. 11 is a chart of plasmon resonance vs. beam angle ofincidence for silver.

[0031]FIG. 12 is an outline perspective view of another cone-shapedembodiment of the present invention.

[0032]FIG. 13 is an outline perspective view of another cone-shapedembodiment of the present invention that includes a magnetic pole.

[0033]FIG. 14 is a chart of effective refractive index for a silvermetal layer sandwiched between two glass layers.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0034] The Appendix describes the science of plasmon waves, inconnection with FIGS. 1 and 2, and provides a theory of operation ofpresent invention. The first embodiment of present invention isillustrated in FIGS. 3a and 3 b.

[0035]FIG. 3a is a perspective view of a plasmon lens 10 that convertsincident light beam 88 into a plasmon wave 90, diffracts the waves at alens surface 17 into a refracted plasmon wave 91 that is focused on aspot 22 located on the a flat surface 23 of the lens 10. At the flatsurface 23, the plasmon wave 91 converts back into light, which may beused to observe a sample in a microscope application, or to heat thesurface of a disc in a magnetic recording application.

[0036] The lens 10 structure consists of a pair of high index dielectriclayers 12, 16 which may be made of SiO₂, SiN, Ta₂O₅, ZnS, TiO₂, Si orother high index materials known in the art sandwiching a thin(typically <50 nm) highly conducting metallic layer 14 which may be madeof gold, silver, aluminum, or copper. The space 18 above the gold layer14 is a dielectric material with a lower refractive index thandielectrics 12 and 16 and may be, for example, air, MgF₂, SiO₂, orAl₂O₃. In one embodiment, the indices of refraction at a wavelength of633 nm are 1.0 for dielectric 18 made of air, 1.5 for both dielectrics12 and 16 made of SiO₂, and 0.183+i(3.09) for a 50 nm layer of metal 14of gold. Referring to FIG. 3b, a light beam 88, which can be a laserbeam, with a wavelength of 633 nm is incident on the gold layer 14 at anangle θ of, for example, 45°. This beam 88 excites a surface plasmon 90at the air/gold interface 20 that propagates towards the right.Essentially all of the incident light beam is coupled into theantisymmetric, leaky mode of the surface plasmon at this angle. Theeffective refractive index, which is the wavevector of the surfaceplasmon normalized by the wavevector of the incident light (2π/λ), forthis mode is 1.05.

[0037] As the surface plasmon propagates to the right, it encountersedge 19 of symmetric glass/gold/glass trilayer structure comprisingupper glass lens 16, the gold layer 14 and the glass substrate 12. Theplasmon wave 90 is here refracted into the plasmon wave 91. This wavecontinues to travel on the surface 25 between the gold layer 14 and theupper glass lens element 16. The effective refractive index for theantisymmetric surface plasmon mode on this surface 25 is 2.35 eventhough the optical refractive index of the glass is only 1.5. Referringstill to FIG. 3b, the thickness h of the upper glass lens element 16 isapproximately 1 μm and may range from 200 nm to 10 μm depending onthicknesses and refractive indices of all materials in the film stack.The lens surface 17 of the upper glass lens element 16 tapers as itapproaches gold layer 14. The greater the taper at this point 19 thebetter so as to make the transition of the plasmon wave 90 into thetrilayer region surface 25 gradual.

[0038] In two dimensions the diffraction limit is slightly smaller thanin three dimensions. The correct equation is, $\begin{matrix}{d = {- {\frac{0.5\lambda}{n\quad \sin \quad \theta}.}}} & (7)\end{matrix}$

[0039] This lens structure 10 for a surface plasmon provides adiffraction limited spot size that is about half that of a glass solidimmersion lens, i.e., ˜135 nm. Furthermore, if the gold layer thicknesst is reduced from 50 nm down to 10 nm in the trilayer region between thelens junction 19 and surface 23, i.e., at surface 23, the effectiveindex of the surface plasmon increases to 4.82. This corresponds to adiffraction limit of 66 nm.

[0040] By increasing the refractive index of the dielectric layers thespot size can be further reduced. The effective index for a surfaceplasmon supported by a 10 nm gold layer between two dielectric layerswith refractive indices of 2 is 9.18, which corresponds to a spot sizein the diffraction limit of 34 nm. This is the regime of interest forHAMR.

[0041] Referring again to FIG. 3a, the electric field amplitude 26 atthe junction of surfaces 23 and 25 is illustrated. This field 26 has amaximum z-axis intensity at spot 22. However, the field 16 drops to its1/e value at 15 nm above and below the center of the gold film, so thesurface plasmon is confined in both the x and z dimensions.

[0042]FIG. 4 is a chart showing the curvature 40 of surface 17 of upperlens element 16. The curvature 40 is derived using the standardprocedures for designing lens curvatures for an SPR SIL lens with alength l (see FIG. 3b) in the y dimension of 1 mm with an origin 0,0 atthe focal point 22 on surface 23. In this case, the wavelength is 633nm, the metal is gold with an initial thickness of 50 nm (and aneffective refractive index of 3.01) and final thickness of 10 nm (andeffective refractive index of 9.18) surrounded by a dielectric with anindex of 2.

[0043] Two issues that must be taken into consideration are (1) thesurface plasmon is lossy, especially at large effective indices, and sowill dissipate heat within the lens 10, and (2) at the junction 19between the two regions 22 and 25 of different effective index there isan impedance mismatch for the surface plasmon and so some energy will bereflected at the junction 19 just as in a standard optical lens. Thiseffect can be minimized by gradually tapering the air/glass junction asillustrated in FIG. 3a, but then the calculation of the necessarycurvature 40 at the junction is more complicated because depending onthe exact shape of the taper, the refraction or bending of the surfaceplasmon will be more or less gradual rather than abrupt.

[0044]FIG. 2, discussed in the Appendix, illustrates a dual-metal layervariation. This dual thickness metallic layer, illustrated in FIG. 2,may replace the single layer 14 shown in the FIG. 3 and may supplementor replace the upper glass layer 16. The two layers, 24 and 26, havedifferent effective indices of refraction depending upon thickness, withthe thinner layer 26 having a higher index of refraction than thethicker layer 24. The interface 19 between the two areas of differentthickness may be curved, as is lens surface 17 illustrated in FIG. 3a,and to form a lens that focuses or assists in focusing the plasmon waveto spot 22.

[0045] Referring again to FIGS. 3a and 3 b, lens 10 focuses the plasmonwave on the flat surface 23 of the trilayer structure (glass layers 12and 16, and gold layer 14) at approximately point 22. There the plasmonwaves convert back into visible light. Without more, this structure isuseful with optical scanning microscopes. It may also be used in HAMR toheat adjacent media. However, in the latter application, it is alsoimportant to locate the magnetic pole used to induce magnetic flux intothe magnetic media as closely as possible to the focus 22 of requiresnear co-location of the optical spot generating heat in the medium withthe magnetic recording pole in order to record rectangular marks withouterasing neighboring tracks. FIGS. 5-7 illustrate several approaches tointegrating such a magnetic pole into lens 10.

[0046] In FIG. 5, a narrow recording pole 50 (composed of a magneticpermeable material such as Permalloy) runs down the central axis of thelens 10. Surface plasmons near this central axis propagating along thegold/dielectric interface 20/25 may be partially absorbed by the lossyrecording pole material. For this reason the pole 50 should generally bekept as narrow, for example, less that 50 nm, as possible while stillallowing a sufficient recording field to be generated within therecording medium. However, surface plasmons 90 which are incident uponthe lens surface 19 away from the central axis are still refracted tothe focal point 22 at the face of the recording pole without beingdisturbed by the pole material.

[0047]FIG. 6 shows a tapered recording pole 50. This recording pole 50structure includes a structure that spans the entire thickness of lens10 towards an anterior portion 52, that narrows through an intermediatesection 54 towards pole tip 50. This tapered pole structure 52, 54conducts more magnetic flux to pole tip 50 without degrading the plasmonfocusing performance of the lens 10.

[0048]FIG. 7 illustrates a half lens embodiment that is more easilymanufactured. This variation eliminates one side of the lens 10, e.g.,to the right of edge 70. Edge 70 is aligned with the right edge of poletip 50.

[0049]FIG. 8 illustrates a second technique for optically excitingsurface plasmons. Here, light 88 propagates through a high index ofrefraction medium 82, such as glass, and is incident upon a planarinterface 85 with a dielectric film 84, such as air, having a lowerindex of refraction, at an angle θ above the critical angle at which thebeam induces plasmon waves. Because the angle of incidence θ0 is abovethe critical angle, the light 88 is totally internally reflected at theinterface, as illustrated. However, this light beam 88 imparts anevanescent field that extends into the dielectric film 84. If a metallayer 86 is brought within range of this evanescent field, a surfaceplasmon 90 is excited by the field at the surface 87 of the metal.

[0050]FIG. 9 is a near field probe 96 that employs the present inventionfor exciting surface plasmons. This “probe” structure has some distinctadvantages for both HAMR and scanning microscope applications.

[0051] This probe 96 is constructed with a layer 86 of a metal likegold, silver, copper or aluminum on the surface of a cone-like cladding99 having an aperture 92. Cladding 99 may be formed of a protectivedielectric material such as glass. The various layers of the probe forman angle Ø at their apexes, illustrated in the FIG. 9 at apex 97 of highdielectric layer 82.

[0052] The thickness of the metal film layer 86 is not critical. Ingeneral it should be sufficiently thick, from about 20 to 50 nm, suchthat no light is transmitted through it. The metal film layer 86 adheresto a thick dielectric film 84 (from about 200 to 800 nm in thickness)with a low index of refraction, which may be anywhere below 1.70. Thisthick dielectric film 84 is in turn coated upon an inner dielectric cone82, such as glass, with a higher index of refraction. The entire probenow consists of three layers: two different dielectrics 82, 84 and ametal film 86, all mounted on a protective dielectric cladding 99.

[0053] A plane wave 88 of light is incident on the probe 96 asillustrated. It propagates within the high index dielectric 82 towardsthe aperture 92. It strikes the high refractive index/low refractivedielectric layer interface 85 at an angle of incidence θ above thecritical angle as illustrated. This excites plasmon wave 90 at the lowrefractive index dielectric film layer/metal layer interface 87. Thesurface plasmon 90 propagates along the inside surface 87 of the metalfilm and has no evanescent tails extending out into the air due to thethick metal film 86. The electric field from plasmon 90 is shielded fromthe microscope sample or the magnetic recording disk until the surfaceplasmon 90 reaches the aperture 92 at the apex of the cladding 99. Theplasmon tunnels through the aperture 92 and emits light radiation 94into the sample adjacent the aperture 92.

[0054] The aperture 92 for HAMR applications may range from 20 to 50 nmin size. For probe applications, the aperture 92 may be as large as 100nm.

[0055] In a specific example, the incident light beam 88 has awavelength of 1000 nm. The refractive index of gold at this wavelengthis 0.257+i(6.82). The inner high index dielectric 82 is chosen to beglass with n=1.5, and the outer low index dielectric cladding 84 ischosen to be MgF₂ with n=1.38 and a thickness of 1000 nm. MgF₂ is acommon material used in optical thin films for antireflection coatings,dielectric mirrors, etc. Referring to FIG. 10, a chart of reflectancevs. angle of incidence of a properly polarized beam of light incident ongold, the plasmon resonance angle is ˜70°. At this angle, thereflectance curve 100 indicates that nearly all of the incident light isabsorbed into creating a surface plasmon. Referring again to FIG. 9, theincident plane wave 88 has an angle of incidence of 70° from the normalto the gold surface. This is the same angle θ that beam 88 is incidenton the high refractive index dielectric/low refractive index dielectricinterface 85. In order for the beam 88 to have a 70° angle of incidenceθ on interface 85, the angle Ø of the probe 96 at its apex 97 is 2·(90°-70°)=40°.

[0056] An advantage of the probe structure is that the outer metal filmcan be coated with a protective dielectric 99 without interfering withthe operation of the surface plasmon dynamics. This in turn allows theuse of silver in place of gold. Silver tarnishes over time when exposedto air and would, therefore, be unsuitable for the a probe designwithout some corrosion protection. Because silver is a much betterelectrical conductor than gold, the fields produced by the surfaceplasmon in silver are larger. Moreover, silver can be used to generatesurface plasmons at much shorter wavelengths than are possible withgold, which in turn enhances the efficiency with which the surfaceplasmon is propagated through the aperture at the tip. Finally, by usingsilver the thickness of the low index dielectric cladding can be greatlyreduced.

[0057]FIG. 11 is a chart of beam reflectance vs. angle of incidence forsilver. The reflectance curve 110 indicates that the silver plasmonresonance angle is ˜79°. This is where nearly all of the incident lightis absorbed into creating a surface plasmon.

[0058] Referring again to FIG. 9, the probe has the following structurewhen silver is used for metal layer 86. Beam 88 has a wavelength of 635nm. This is a common wavelength available from semiconductor lasers. Therefractive index of silver at this wavelength is 0.135+i(4.00). Theinner high index dielectric 82 is again glass with n=1.5. The outer, lowindex dielectric cladding 84 is again MgF₂ with n=1.38 and a thicknessof 400 nm. In accord with FIG. 11, the resonance angle is ˜79°. Thisthen is the angle of incidence θ of beam 88 on interface 85. The apexangle Ø of the probe 96 is ˜220.

[0059]FIG. 12 illustrates a probe 120 having rectangular cone structureshown without a protective cladding 99 for the sake of clarity. Arectangular cone provides compatible geometry because the angle ofincidence at the surface of the cone is a constant for a plane waveentering the top of the cone along the cone axis. Only the polarizationcomponent 89 of the incident beam 88 that is parallel to the plane ofincidence (the TM or p-rectangular cone provides the most cone surfacearea as possible to receive this polarization of the incident lightbeam.

[0060] In FIG. 12, the outer metal layer 86 is composed of either goldor silver. The low index of refraction dielectric layer 84 is preferablycomposed of MgF₂. The central, high index of refraction layer 82 iscomposed of glass. Incident light beam 88 preferably is polarized asshown with the p-polarization 89 parallel to the elongated interface 85between the glass 82 and MgF₂ layers 84. Alternatively, the incidentlight beam 88 may be infrared radiation. In this case, the high indexlayer 82 could be silicon with a refractive index of 3.6 at a wavelengthof 900 nm. A wide range of materials, such as SiO2 and SiN, may thenavailable for the low index dielectric layer 84.

[0061]FIG. 13 illustrates a variation of the present invention for HAMR.For this application, a magnetic recording pole must be co-located withthe source of near field radiation. A geometry and that would co-locatethe probe and recording pole is illustrated in FIG. 13. In thisstructure, the functioning probe structure, dielectric layers 82 and 84,and metal layer 86, uses only half of a cone located adjacent to arecording pole 130.

[0062] The configuration of this embodiment is identical to that of FIG.12, with the addition of magnetic pole layer 130 replacing the metallicand low index of refraction dielectric layers, 86 and 84, along onemajor surface of the rectangular cone. The magnetic pole 130 terminatesin pole tip 104 adjacent to the aperture 92 that emits light from theplasmon waves.

[0063] The above description of the preferred embodiments is not by wayof limitations on the scope of the appended claims. In particular, thoseof ordinary skill in the art may substitute other materials for thedisclosed materials and other focusing structures than those describedhere. For example, copper or aluminum may generally replace gold orsilver in the preceding examples.

I claim:
 1. Apparatus for focusing plasmon waves comprising: a metalliclayer; a first dielectric layer having a first index of refraction; asecond dielectric layer having a second index of refraction higher thanthe first index of refraction; and the metallic layer and the first andsecond dielectric layers arranged to focus to a spot plasmon wavesinduced at the interface between the metallic and first dielectriclayers in response to light incident on the second dielectric layer. 2.Apparatus according to claim 1 wherein said metallic layer has astructure that converges to an aperture and wherein said spot isco-located with said aperture.
 3. Apparatus according to claim 2 whereinsaid metallic layer and said first and second dielectric layers form arectangular cone.
 4. Apparatus according to claim 3 wherein one side ofsaid rectangular cone further comprises a magnetic recording pole. 5.Apparatus according to claim 1 wherein said metallic layer is selectedfrom a group comprising gold or silver or aluminum or copper. 6.Apparatus according to claim 1 wherein said first dielectric layer isselected from a group consisting of air, MgF₂, SiO₂, SiN or Al₂O₃. 7.Apparatus according to claim 1 wherein said second dielectric layer isselected from a group consisting of SiO₂, Al₂O₃, silicon, SiN, Ta₂O₅,TiO₂, GaP or ZnS.
 8. Apparatus according to claim 1 further including aprotective layer affixed to said metallic layer opposite said firstdielectric layer.
 9. Apparatus according to claim 4 wherein saidmagnetic recording pole is composed of a magnetically permeablematerial.
 10. Apparatus according to claim 1 wherein: said metalliclayer comprises a thin film; said first dielectric layer is arranged ona first side of said metallic layer; and said second dielectric layer isarranged on the opposite side of said metallic thin film from said firstdielectric layer.
 11. Apparatus according to claim 10 furthercomprising: a third dielectric layer arranged on the same side of themetallic layer as the first dielectric layer, the third dielectric layerhaving a third index of refraction greater than said first index ofrefraction on the first dielectric layer, the third dielectric layerhave a curved surface that is arranged to focus the plasmon waves onsaid spot.
 12. Apparatus according to claim 11 further comprising amagnetic pole layer arranged in a layer perpendicular to said metalliclayer; said magnetic pole layer running through said spot.
 13. Apparatusaccording to claim 12 wherein said metallic layer, said third dielectriclayer and said magnetic pole all terminate in a third, substantiallyflat surface running through said focal point spot.
 14. Apparatusaccording to claim 13 wherein said metallic layer and said first, secondand third dielectric layers are all arranged on only on side of saidmagnetic pole layer.
 15. Apparatus according to claim 10 wherein themetallic layer has a region of different thickness arranged on the sameside of the metallic layer as the first dielectric layer in the path ofinduced plasmon waves, the thinner metallic region having an index ofrefraction higher than the thicker metallic region, the interfacebetween the two regions having a curvature that is arranged to focus theplasmon waves on said spot.
 16. Apparatus for focusing plasmon waves ona spot, comprising: means for inducing plasmon waves on a metallicsurface; and means for focusing the plasmon waves to a spot. 17.Apparatus according to claim 16 further including, means for convertingthe plasmon waves at the spot into light.